1. Using an electric mixer, beat butter in a large bowl until very pale, about 2 minutes. Reduce mixer speed to low and gradually add confectioner's sugar, beating until fully incorporated. Continue beating on a slow speed for 2 minutes. Add vanilla extract.
2. Using a food processor, pulse freeze dried strawberries or raspberries to a fine powder. Sift over frosting mixture with a fine mesh sieve and discard any seeds. Fold into frosting along with milk.
3. If necessary, trim rounded tops off cakes to make flat layers. Place one cake layer on a serving dish and spread with 1/2 cup frosting. Top with second cake layer, bottom-side up. Use a 1- to 1 1/2-inch heart-shaped cookie cutter to stamp out 3 rows of 3 hearts from the top cake layer and carefully remove.
4. Fill a re-sealable bag with remaining frosting and snip off corner. Pipe evenly into stamped-out hearts. Spoon in sprinkles if desired, about 1 tsp. per heart.